Silver rhenium electric contacts



y May12,1970 J. SCHMIDT, JR. 3,511,953

SILVER RHENIUM ELECTRIC CONTACTS Filed June 6, 1968 United States PatentO 3,511,953 SILVER RHENIUM ELECTRIC CONTACTS John Schmidt, Jr., MountProspect, Ill., assigner to Guardian Electric Manufacturing Co.,Chicago, Ill., a corporation of Illinois Filed June 6, 1968, Ser. No.735,108 Int. Cl. H01h 1/02 U.S. Cl. 200-166 5 Claims ABSTRACT OF THEDISCLOSURE A uniform dispersion of 5-l5% by weight rhenium in a silvermatrix provides an electric contact which ex- BACKGROUND OF THEINVENTION This invention relates to an improved electric contact and, inparticular, an electric contact fabricated from a dispersion of rheniumin silver.

Silver has long been recognized as an excellent conductor andconsequently a good material for electric contacts especially at lowcurrent ratings. However, for higher current ratings, silver contactserode or fuse when circuits are made and broken. A notable advance inthe art of electric contacts was the discovery of silver cadmium oxideas a contact material. Silver cadmium oxide provides excellent currentcarrying characteristics and is one of the more common high currentcarrying contact materials.

Other pure metals, alloys and dispersions have been considered ascontact materials also. An example of a patentable combination of metalsfor a contact material is disclosed in U.S. Pat. 2,234,969 issued Mar.18, 1941 to Hensel et al. There tungsten in combination with variousrare earth borides is disclosed as a contact material. Tungsten, alone,is also a well known and durable pure metal contact material. Tungstenis extremely difficult to work however.

Rhenium is still another pure metal which has been considered as acontact metal. For example, U.S. Pat. 2,733,319 issued Ian. 31, 1956 toEricsson et al. discloses a method of producing an electric contactmaterial made of pure rhenium. Ruskin in U.S. Pat. 2,997,524 issued Aug.22, 1961 teaches the use of a pure rhenium coating approximately l mm.in thickness over a copper conductor base for a contact material. Theuse of rhenium as a contact material has been very limited, however,because of the great cost of rhenium (approximately $50.00 per ounce)and the difficulty of working the metal.

Thus, while the art of electric contacts has been extensivelyinvestigated, an improved contact is desirable. An improved contactshould be available at a cost comparable to the cost of presentcontacts. In addition an improved contact as compared with presentlyavailable contacts should be capable of carrying greatly increased loadsfor increased periods of time without failing. Finally, an improvedcontact should be capable of being fabricated on currently availableequipment using currently known methods.

SUMMARY OF THE INVENTION In a principal aspect the present invention ofan irnproved contact for electric switches, relays, contactors, etc.consists essentially of approximately 5-15% by ice weight rheniumdispersed in silver, silver being the matrix. Preferably the rhenium isuniformly dispersed as a particulate, with the particles each being lessthan microns in diameter. The majority of particles are in the rangefrom l0 to 25 microns in diameter.

It is thus an object of the present invention to provide an improvedelectric contact.

It is a further object of the present invention to provide a silverrhenium electric contact having improved electric characteristics incomparison to prior art contacts.

Still another object of the present invention is to provide an electriccontact which reduces contact size requirements as compared to prior artcontacts and at the same time carries current loads equal to or greaterthan those carried by prior art contacts.

One further object of the present invention is to provide a contactwhich may be provided at a cost competitive with presently availablecontacts having the same current carrying characteristics.

Still another object is to provide a contact which may be fabricatedwith presently available contact fabrication equipment and methods.

These and other objects, advantages and features of the presentinvention will be set forth in greater detail in the description whichfollows.

Brief description of the drawing In the detailed description whichfollows, reference will be made to the drawing comprised of thefollowing figs.:

FIG. 1 is a pictorial representation of a photomicrograph of alongitudinal axis cross section of a wire having a dispersion of rheniumin the range 5l5% by weight in a silver matrix;

FIG. 2 is a pictorial representation of a photomicrograph of alongitudinal axis cross section of a round head electric contact madefrom wire of the type shown in FIG. l;

FIG. 3 is a pictorial representation of a photomicrograph of alongitudinal axis cross section of a flat head contact made from wire ofthe type shown in FIG. 1; and

FIG. 4 is a pictorial representation of a photomicrograph of a crosssection transverse to the longitudinal axis of the wire shown in FIG. l.

Description of the preferred embodiments The contacts of the presentinvention are prepared by standard powder metallurgy techniques. Thus,rhenium powder is supplied having a particle size equal to the naldesired size of particles in the dispersion. The metal powders arethoroughly mixed and then fabricated into billets, wire or rod in amanner similar to that used for fabricating powdered tungsten intovarious useful struc.V

tural shapes. In mixing and fabricating the sintered material,composition is maintained within 1%, e.g. 10% i1% rhenium in silver.

Thus, the mixed powder is rst pressed by hydraulic means, for example,into a billet. The billet is then sintered at a relatively lowtemperature so that the silver and rhenium particles comprising thebillet collease and provide some structural strength to the billet. Thesintering operation preferably takes place in an inert atmospherefurnace.

Next, the low temperature sintered billet, is sintered at an elevatedtemperature below the melting point of silver. This may be done, forexample, by electrical resistance means in a protective atmosphere.During the second sintering operation, the density of the silverrheniumdispersion is greatly increased. This is facilitated by the wettingaction of the silver which tends to W about the rhenium particles andlill the interstices between the rhenium particles. The finished productis a uniformly dispersed particulate of rhenium in a silver matrix. Thedensity of the mixture should be in excess of 90% of the theoreticalweight per unit of volume for the combination of silver and rhenium.

The billet is then Worked by various methods such as forging, swaging ordrawing to reduce it to proper size rod or wire. Rod or wire is, ofcourse, the structural shape from which electric contacts are mostcommonly made. The swaging operation and other working operationsrequire annealing steps between each operation. This results because thedispersion work hardens. Pure rhenium work hardens also to a muchgreater degree than the dispersion of the present invention. For thisreason pure rhenium is almost impossible to cold work. By contrast, thematerial of the present invention may be cold Worked for at least onepass without adverse effects to tools or the like.

Following preparation of wire or rod of sufficient size for the desiredcontact, the Wire or rod is formed into a contact. The methods forfabricating contacts are well known to those in the art. For example,cold heading of pre-annealed silver-rhenium wire may be practiced usingconventional tools and equipment. Another procedure by which contactsmay be formed from the present material is the well known coiningmethod.

AReferring now to FIGS. 1, 2, 3 and 4 there is shown pictorialrepresentations of a dispersion in the range 5 by weight rhenium in asilver matrix. The rhenium is depicted Iby the dark ink within theoutline of the wire or contact. First FIG. 1 shows a pictorial view of atypical photomicrographed longitudinal cross section of the rheniumsilver Wire of the present invention. FIGS. 2 and 3 illustratephotomicrographs of round head and iiat head contacts respectivelyfabricated from rhenium-silver wire of the type shown in FIG. 1. Thelongitudinal axes of the wire in FIG. l and the contacts in FIGS. 2 and3 are aligned in the same directions for the purposes of comparison.FIG. 4 illustrates a typical cross sectional view of a photomicrographof the the wire shown in FIG. 1. FIG. 4 may also be considered a crosssectional view of the contacts shown in FIGS. 2 and 3 sincephotomicrographs of cross sections of the contacts are substantiallyidentical to photomicrographs of the wire cross section.

As illustrated in the figures, the rhenium particulate is uniformlydispersed in the silver matrix. In addition the silver matrix tends towet and adhere to the particles of rhenium. The rhenium particle sizesare normally less than 100 microns in diameter with the mean range ofdistribution being 10 to 25 microns in diameter. No clumping of theparticles is observed. Moreover, it is believed that clumping isundesirable.

As evidenced by reference FIG. 1, the swaging and wire drawingoperations used to fabricate the wire, tend to cause the rheniumparticles to align themselves along flow lines coincident with thelongitudinal or draw axis of the wire. After the contacts are formed bythe cold heading operation as illustrated in FIGS. 2 and 3, the ow linesof the material indicate the manner in which the metal is deformed whencold heading, which is a forging operation.

The contacts are used in electric switches. Switches as called for inthe claims, include any electric device which utilizes contacts orcontact material to make and break electric circuits. For example,relays and contactors are switches within the scope of the claims.

Various tests were conducted with contacts of the present invention. Theresults were compared to the results of similar tests conducted withpure silver and silver cadmium oxide contacts, a well known high currentcarrying contact material. Where possible comparison data with purerhenium is supplied. In many instances, however, such data isunavailable or almost impossible to determine because rhenium is veryhard and extremely dicult to work. Moreover, the electricalcharacteristics of rhenium make it impossible to run many of the testsoutlined below. These tests are -well known in the art and form part ofmost military specications for relays, switches, etc. In the tests, allof the contacts were approximately equal in size and were examined undersubstantially identical conditions.

Test results One double headed, rounded or radius faced contact with adiameter of approximately 0.125 inch and a thickness of approximately0.031 inch and a flat faced contact of similar dimensions were testedfor current interrupting ability a. Initial contact resistance b,minimum current switching c and maximum load carrying d were tested with0.187 inch diameter, 0.038 inch thick contacts. These contacts wereattached to copper alloy blades of such dimensions to permit a nominalrating of the composite assembly of l0 amperes. The results werecompared with results from similar tests on equally sized silver, silvercadmium oxide and, where possible, rhenuim contacts.

silver,

0 silver, cadirnum Silver Rheniurn 10% rhenium oxide Test (a) .-CurrentInterrupting Ability. In this test a high current circuit isinterrupted. The amount of current and the number of interruptions ormaking and breaking of a circuit under load are maximieed:

Spacing between contacts ins 0. 022 0. 022 0. 022 Load ln amperes 50 (l)100 50 Number of operations (i) At least 2 50 50 max. Test (b) .-InitialContact Resistance. In this test the voltage drop for a given currentthrough the contacts is determined. Preferably a low voltage drop isobserved' Current in ma-- 5 500 500 Voltage drop in mv (3) 0. 60-0. 750. 55-9. 70 Test (c).-Minim1nn Current Switching. Change in millivoltdrop for a milliamp load following 100,000 switching cycles: 4

Initial reading in millivolts 1. 0-1. 5 1. 5-2. 0 2. 0-2. 5 Finalreading in mi1livolts 1. 0-1. 5 2. 0-3. 0 6 0-7. 0 Charge in millivolts0 (5) 0. 5-1. 5 3 5. 0 Test (d).-Maximum Load Test. The

maximum current in amp eres resulting in failure of the contacts -200(5) 250 -220 1 Not tested.

2 Alter fifty (50) interruptions the test was halted. No appreciableerosion 0i the contacts Was noted and it is believed that innumerableinterruptions may be made.

S The voltage drop for rhenium is a variable and can reasonably beexpected to be 10-20 times that of silver.

The normal maximum allowed change is 7.0 mv.

5 This data is impractical to obtain.

Similar tests carried out on a 97% by weight silver- 3 by weight rheniumdispersion produced no significant improvement over silver cadmium oxideor silver. However, the 3% rhenium dispersion as tested was not asuniform as dispersion. Nevertheless, it is believed that the differencescannot be attributed entirely to the difference in uniformity ofdispersion and particle size that was observed. -For this reason it isbelieved that rhenium dispersion in the range 5-15% by weight provides asubstantial improvement in contact characteristics not heretoforeobserved and that the improvement is related, among other factors, tothe amount of rhenium in the dispersion. Besides having the ability tocarry larger loads and interrupt a wider range of loads, the contacts ofthe present invention compare favorably in cost with prior art contactsand also may Ibe fabricated using standard contact manufacturing methodsand equipment.

What is claimed is:

1. In an electric switch the improvement comprising at least oneelectric contact consisting essentially of rhenium in the range of 5-15%:by weight and the remainder silver, said silver comprising a matrix inwhich said rhenium is dispersed.

2, The improved contact of claim 1 wherein said rhenium is approximately10% by weight of said contact and said silver is approximately by Weightof said contact.

3. The improved contact of claim 1 wherein said rhenium substantiallyuniformly dispersed in said matrix.

4. The improved contact of claim 1 wherein said rhenium is a particulatewith the diameter of each of said rhenium particles being less than lmicrons.

5. The improved contact of claim 1 wherein said rhenium is a particulatewith the diameter of each of said rhenium particles being in the rangeof 10-25 microns.

References Cited UNITED STATES PATENTS 2,234,969 3/ 1941 Henel et a1.2,733,319 1/1956 Ericsson et al. 2,861,155 ll/l958 Farnham etal.2,914,640 11/ 1959 Grattidge. 2,997,524 8/1961 Ruskin et al.

HERMAN O. JONES, Primary Examiner

